1. Field of the Invention
The present invention relates generally to an apparatus and method for canceling interference from neighbor cells in a broadband wireless communication system, and in particular, to an apparatus and method for canceling interference from neighbor cells in a broadband wireless communication system using multiple receive (RX) antennas.
2. Description of the Related Art
A broadband wireless communication system uses a cellular system that divides the service coverage area into a plurality of small areas (i.e., cells) in order to overcome restrictions in a service coverage area and subscriber capacity. In the cellular system, two cells spaced apart from each other by a sufficient distance use the same frequency band, making it possible to reuse a frequency resource spatially. Accordingly, the cellular system increases the number of channels distributed spatially, thereby making it possible to secure a sufficient number of subscribers. However, a wireless communication system with a frequency reuse factor of 1 has a problem in that RX performance is degraded by interference between neighbor cells.
FIG. 1 is a diagram illustrating the structure of a general broadband wireless communication system.
Referring to FIG. 1, a mobile station (MS) 104 is located in the cell coverage of a serving base station (BS) 100 and communicates with the serving BS 100.
If the MS 104 moves into a handoff region (i.e., a cell boundary region), it receives not only a target signal 111 from the serving BS 100 but also an interference signal from a neighbor BS 102, as expressed in Equation (1):y(k)=hs(k)·xs(k)+hl(k)·x·(k)+n(k)  (1)where hs(k) is a channel corresponding to the kth subchannel between the serving BS 100 and the MS 104, hl(k) is a channel corresponding to the kth subchannel between the neighbor BS 102 and the MS 104, xs(k) is a signal received from the serving BS 100 over the kth subchannel, xl(k) is a signal received from the neighbor BS 102 over the kth subchannel, and n(k) is an Additive White Gaussian Noise (AWGN) corresponding to the kth subchannel.
The interference signal received from the neighbor BS 102 causes a degradation in the RX performance of the MS 104. Research has been conducted to provide methods for reducing the RX performance degradation due to the interference signal in the broadband wireless communication system. For example, the MS 104 disregards the interference signal and detects the target signal. For example, the MS 104 cancels the interference signal using a Successive Interference Cancellation (SIC) scheme or a Parallel Interference Cancellation (PIC) scheme.
If the MS 104 is near to the serving BS 100 and far from the neighbor BS 102, the power of the interference signal received by the MS 104 is much smaller than that of the target signal. The MS 104 disregards the interference signal in a received signal expressed in Equation (1) to estimate and detect the target signal as expressed in Equation (2):{tilde over (x)}s(k)=ĥs−1(k)·y(k),{circumflex over (x)}s(k)=Q({tilde over (x)}s(k))  (2)
where {tilde over (x)}s(k) is an estimated target signal component, hs(k) is a channel component of the serving BS 100 and the MS 104, y(k) is the received signal, and {circumflex over (x)}s(k) is a detected target signal component.
However, when the MS 104 is located in a cell boundary region between the serving BS 100 and the neighbor BS 102, an interference signal and a target signal received at the MS 104 are similar in power. Accordingly, when the MS 104 disregards the interference signal to estimate and detect the target, signal performance degradation occurs due to the interference signal.
In the SIC scheme, the MS 104 estimates the interference signal and cancels the estimated interference signal from the received signal to detect the target signal. For example, using Equation (3) below, the MS 104 estimates a channel component between the neighbor BS 102 and the MS 104 to restore the interference signal.{tilde over (x)}l(k)=ĥl−1(k)·y(k),{circumflex over (x)}l(k)=Q({tilde over (x)}l(k))  (3)
where {tilde over (x)}l(k) is an estimated component of the interference signal, hl(k) is a channel component of the neighbor BS 102 and the MS 104, y(k) is the received signal, and {circumflex over (x)}l(k) is a detected target signal component.
Thereafter, by applying the restored interference signal to Equation (4) below, the MS 104 cancels the interference signal from the received signal to restore the target signal.w(k)=y(k)−ĥl−1(k)·{circumflex over (x)}l(k)=hs(k)·xs(k)+n′(k)  (4)
where {circumflex over (x)}l(k) is the detected interference signal, hl(k) is a channel component the neighbor BS 102 and the MS 104, and y(k) is a received signal.
As expressed in Equation (4), the MS 104 cancels a neighbor signal component from the received signal and estimates and detects the target signal using the channel component of the serving BS 100.
If there are several interference signals, the MS 104 arranges the interference signals according to Carrier to Interference and Noise Ratios (CINRs), restores the interference signals serially, and cancels the restored interference signals from the received signal.
The PIC scheme restores an interference signal in the same way as the SIC scheme and cancels the restored interference signal from a received signal to detect a target signal. Unlike the SIC scheme, if there are several interference signals, the PIC scheme restores and cancels the interference signals in parallel.
As described above, the SIC scheme and the PIC scheme are interference cancellation schemes that restore the interference signal, cancel the restored interference signal from the received signal, and detect the target signal. However, when the interference signal is restored, the SIC scheme and the PIC scheme ignore the target signal, increasing the error probability of the restored interference signal. When the MS detects the target signal using the interference signal in which the error occurs, the error propagation is caused, degrading system performance.
In addition, the MS must find the Modulation and Coding Scheme (MCS) level of the interference signal in order to restore and cancel the interference signal. Therefore, the MS must demodulate and decode the interference signal. Consequently, the time delay occurs in the MS in order for the interference cancellation. Thus, the size of the buffer in the input terminal increases, causing an increase of hardware complexity.